The present invention relates, in general, to a method and apparatus for surveying generally horizontal boreholes below the earth""s surface, and more particularly to a system for detecting and precisely locating a drill head in a borehole with respect to a known location, for use in guiding the drilling of the borehole to a specified location.
Horizontal directional drilling techniques are well known, and have long been used to drill boreholes which cross under areas where trenching is not permitted or is impractical. For example, such techniques are used to drill boreholes under manmade or natural obstacles, such as bodies of water, rivers or lakes, and under highways, airport runways, housing developments, or the like. These boreholes may be used, for example, to position pipelines, underground transmission lines, communications lines such as optical fibers, and other utilities, and often must be drilled within defined areas, must travel long distances, and must exit the ground at predetermined locations.
Conventional directional drilling techniques used to drill such boreholes commonly use a steering tool which measures the borehole inclination, azimuth and tool roll angle at each station where measurements are made. The borehole coordinates are computed and tabulated from these steering tool data as a function of the measured distance along the borehole, which may be referred to as the measured depth of the steering tool. These borehole coordinates suffer from serious cumulative effects caused by the inclination and azimuth determinations made at regularly spaced stations along the borehole, and the lateral errors generated by such conventional borehole surveying are intolerable. The inherent imprecision of this integration is the reason for turning to electromagnetic methods for directly determining drill bit location. However, determination of the radial away distance from the entry point to the drill bit is quite precise since a borehole normally changes direction slowly and modestly in both inclination and azimuth along its length. Thus, if a borehole has been following a curved borehole proposal design path, and has not deviated by more than 3 degrees in direction from the design path direction for 500 meters of drilling, the lateral error with respect to that design proposal could be 25 meters, whereas the radial away error would be less than 1 meter. Accordingly, the present invention incorporates the inherent precision of the radial away distance and the use of electromagnetic processes for determining lateral position in order to precisely locate a drill bit.
important aspect of drilling boreholes for pipeline and cable burial projects is the requirement that the borehole exit at the Earth""s surface at a precisely determined location. In order to do this, the driller not only must have a direct determination of lateral position, but also needs reaffirmation of the precise radial distance to the exit location at a distance from that exit point so that appropriate adjustments to the inclination of drilling can be made. Even if the radial distance to the exit location from the entry point of the borehole into the Earth is precisely known and the radial distance of the drill bit from its entry point into the Earth is precisely known, safety considerations alone give high priority to directly determining the relative location of the desired borehole exit point with respect to the drill bit location as the exit point is approached. This invention discloses improved methods of guiding the drilling to the required exit location.
A further important concern in drilling is responding to a sudden and unexpected deflection of the borehole by up to several degrees due to hitting boulders or other obstacles. Immediate correction of such a drilling direction perturbation can be more important than immediate correction of a displacement error since such direction perturbations can lead to a tortuous borehole, which is a very serious defect particularly when attempting to pull a pipeline through the completed borehole. Steering tool inclinometers provide good inclination measurements, usually to a precision of 0.1 or 0.2 degrees; thus good control exists in inclination. However, the standard steering tool azimuthal direction determination provided by the Earth Field magnetometers is inadequate. In addition to being intrinsically much less precise than the inclinometers because of steel in the drill string, motor and drill bit, they are also subject to sudden environmental changes from steel and magnetized objects in the vicinity of the borehole and by nearby auto, truck, train, and ship traffic. This invention provides a much needed method and an apparatus for measuring drilling direction perturbations.
A variety of attempts to improve the accuracy of underground drilling have been made. One such attempt included the use of grids on the earth""s surface to guide the drill head, but if access to the surface is not available along the length of the borehole, this technique can encounter serious problems. For example, electrical current-carrying surface grids may be placed on both sides of a river, but since such grids have a limited range, they may not be effective if the borehole drifts away from its planned path as it travels from one grid to the other. Other attempts have included the use of two-loop antenna systems for generating two fields with different frequencies, which are measured by magnetometers mounted within the drill head. Still other attempts to provide improved drill guidance include the use of an externally generated magnetic field produced by one or more current loops made up of straight line segments, wherein the fields are measured by a probe at the drill, the probe having three orthogonal magnetometers which measure X, Y and Z components of the magnetic field. Three accelerometers measure the rotation of the probe with respect to gravity, and this data is used to determine the magnetic field vector at the magnetometers. A theoretical magnetic field vector is then calculated and compared to the measured vector to determine the location of the probe.
Although some of these prior systems have been adequate for many applications, they have not been totally satisfactory, and there exists a need for an improved borehole surveying method which will permit accurate and reliable location of drill heads for drill heads to enable boreholes to be drilled along preselected paths to distant locations.
In accordance with preferred embodiments of the invention, improved methods for precisely surveying the path of a borehole in the Earth are provided. These methods are not only used to locate a drill head and its steering tool in a borehole in order to provide data for guiding the drilling of the borehole along a prescribed path, for example to an exit point at a remote location, but may also be used for other purposes such as surveying existing boreholes.
The method of the present invention is based on the use of a detector which may be adapted from, or which may be similar to, those which are found in conventional steering tools for drill assemblies. In a preferred form of the invention, the detector incorporates two single-axis electromagnetic field sensors which preferably are perpendicular to each other and to the axis of the steering tool, and lie on an imaginary xe2x80x9cpatchxe2x80x9d, or segment on the surface of a sphere. The sensors are approximately perpendicular to the radius of this spherical segment, with the radius being centered at a fixed location from which measurements are to be made; for example, at the entry point of a borehole being drilled into the Earth. One or more guide wires are located on the Earth""s surface near the borehole entry point and/or the borehole exit point, and extend along the surface above the prescribed borehole path. An electromagnetic field is generated in the Earth in the region of the prescribed path by a known electric current flowing through the guide wires. Values of x and y vectors of this electromagnetic field at multiple locations on a spherical surface at the radial distance where the sensors are known to lie are calculated, and the x and y vectors at that location are measured by the two sensors. The location on that spherical surface where the calculated magnetic field values equal the measured magnetic field values defines the location on the patch where the sensors lie, and permits determination of the lateral spacing between the sensors and the desired path of the borehole.
An example of the use of the foregoing method is in the guidance of a conventional drill assembly, including a drill bit carried by a drill stem, to drill a borehole from an entry point at a near side of an obstacle, such as a river, under that obstacle to an exit, or punch-out, location at the far side of the obstacle. The drill assembly includes a drill stem having a conventional steering tool which carries two single-axis electromagnetic sensors which are perpendicular to the steering tool axis. Drilling apparatus at the entry point includes conventional guidance equipment for receiving data signals from the steering tool and for providing suitable control signals for regulating the direction of drilling. One or more guide wires, such as electrical current-carrying source loops, are positioned on the Earth""s surface along the proposed path of the borehole to produce corresponding electromagnetic fields along that path. A first loop may be located on the near side of an obstacle, with a part of this loop being located near the entry point, and a second loop may be located on the far side of the obstacle, with a part of the second loop being located near the punchout point. The locations of these loops are known, since their coordinates are determined by conventional land surveys. The first loop is used to guide the drilling at the near side of the obstacle, and the second loop guides the drilling at the far side, with drilling under the obstacle, where the electromagnetic fields from the surface loops are not available, being guided using conventional survey guidance techniques. A pair of loops may be provided at the far side, in another embodiment of the invention.
The borehole entry point preferably is used as the fixed location from which is determined the radius of the surface segment of the sphere, or patch, on which the sensors lie. This radius is the straight line distance, or a vector, between the sensor patch and the entry point, and is very close to the radius determined from integration of the standard steering tool measurements of inclination from the Earth""s gravity and azimuth from the Earth""s magnetic field along the borehole to the measured depth of the sensors. In many cases, when the borehole is almost straight, this radius is effectively equal to the measured depth of the sensors. The imaginary spherical patch is perpendicular to the radius which is centered at the entry point, and is usually almost perpendicular to the axis of the steering tool which carries the sensors. This radial distance may be referred to as the xe2x80x9caway distancexe2x80x9d of the sensors. Where this away distance is large, the patch on the spherical surface where the sensors are located is effectively planar.
The method of the invention, in which measured x and y components of a magnetic field are used in conjunction with a radial distance measurement to locate a bore hole with respect to a planned path may be carried out using a variety of guide wire configurations. For example, an electromagnetic field source loop having arbitrary, but known configuration coordinates with respect to the borehole entry point into the Earth generates a calculable magnetic field at any point on a spherical patch on which the sensors lie. A single measurement by each of the two field sensors at a single measuring site at a known radius will then be sufficient to determine the location of the sensors by comparing the measured values to the calculated values, and this will provide sufficient information to provide directional control of the drill.
Where the relative coordinates of a source guide wire loop with respect to a distant reference point, such as the punch-out location discussed above, are known, but the exact distance to that punch-out location from the fixed location of the center of the sphere; i.e., the entry point, is not known, then additional information is needed to determine the drilling direction to the punch out location. Two embodiments of the invention are available for providing this additional information and for determining the distance between the two points.
In the first embodiment, the surface loop near the punch out location is configured so that the current flow in the loop produces a rapidly changing electromagnetic field at some region along the path of the borehole; for example, as the borehole approaches an edge of the surface loop. Two sets of measurements are made by the two sensors, one set at each of two closely spaced measuring sites, or depths, in the borehole in the region where the electromagnetic field is rapidly changing. This results in four field measurements taken at the two measured depths, or away distances, in the borehole. These are compared and matched to theoretical field values which are computed for spherical segments at the two depths. Over the short borehole depth interval between the two sets of measurements, the relative lateral locations of the sensors with respect to the surface loop can be determined by a straight line projection of the borehole. As a result, the radial distance between the sensors and the planned punch-out location of the borehole, in addition to the lateral location of the borehole, can be determined from the change in electromagnetic field components between the two sites.
The second embodiment of the method of the present invention utilizes two electromagnetic field source loops, near the punch-out location, which are independently excited and which are configured to produce a rapidly changing electromagnetic field at neighboring regions or locations in the borehole. Two sets of measurements are made by the two electromagnetic field sensors at a single borehole site for fields generated by each of the loops. These four field measurements are matched to four computed values for that site to obtain the radial distance to the proposed punch-out location in addition to the lateral location information for the drill with respect to the planned path of the borehole.
For a guide wire drilling method, such as the one disclosed herein, the electromagnetic field vector at the borehole is usually dominantly perpendicular to the borehole over most of its length. If the radial location of the electromagnetic field sensors from the borehole entry point into the Earth is already known, the measurement and matching all three orthogonal (x,y, and z) vector components of the electromagnetic field at field sensors in the borehole to computed values to determine borehole location is basically bad, for the axial (z) component of the field at the drill bit has more to do with the orientation of the borehole than its location. The difference between field vector components measured by x and y sensors perpendicular to the borehole and the true components which are approximately perpendicular to the borehole is small, for a 5 degree error in borehole direction results in only a 0.4% error in the measurements while a 9.0% error is generated in the axial component. Thus, the dominant effect of adding a measurement of the 3rd component, i.e., the axial component, to the field matching procedure while determining lateral location is the introduction of error.
The z, or axial component measurement can, however, be used to provide vital azimuthal drilling correction information. To the extent that the total electromagnetic field vector is dominantly perpendicular to the borehole, the axial vector is proportional to the sine of the non-perpendicularity of the field and the borehole, i.e., the axial vector is dominated by the borehole orientation rather than its location. Modeling the axial component of the electromagnetic signal together with the inclination determination provided by steering tool measurements can provide the information needed to make an azimuthal determination of borehole direction.